Proteins that contain copper in their active sites represent a large and functionally significant class of metallobiomolecules that play central roles in life processes. Electro transfer, reversible binding and activation of dioxygen, oxidation of organic molecules, and nitrogen oxide activation are illustrative examples of the wide range of important reactions performed by copper proteins. This functional diversity is matched by a high degree of variability in the geometric, electronic structural and spectroscopic features of the copper active sites. Despite extensive efforts to relate these features to functional attributes through experimental and theoretical studies, our understanding of structure/function relationships at the molecular level is incomplete and many questions concerning the detailed mechanisms of copper-mediated processes in biology remain unanswered. Such issues will be addressed in the proposed research through the synthetic modeling approach, wherein low molecular weight complexes designed to replicate metalloprotein active site structure and function are characterized and their reactivity examined. Through this approach, fundamental chemical insights into copper site structure and mechanisms of action will be obtained. The specific aims of the proposed research are to: 1.Understand the mechanisms of nitrogen oxide processing by copper proteins such as nitrite and nitrous oxide reductase, important players in the global nitrogen cycle. Toward this end, the synthesis , characterization, and reactivity of copper-hyponitrite complexes will be targeted. 2.Understand how structural perturbations influence the function of the ubiquitous copper-thiolate electron transfer sites. In particular, the synthesis and characterization of models of perturbed mononuclear type 1 and dinuclear, mixed-valence CuA centers will be pursued. 3.Obtain fundamental chemical information on the pathways of dioxygen activation at mononuclear copper sites such as those found in dopamine-beta-monooxygenase and peptidylglycine alpha hydroxylating monooxygenase, important catalysts in mammalian hormone biosynthesis. The synthesis, characterization , and reactivity of new monocopper-dioxygen species will be studied. 4.Understand dioxygen activation at trinuclear copper sites found in the multicopper oxidases and particulate methane monooxygenase by targeting the synthesis and O2 reactivity of tricopper(I) complexes of preorganized N-donor ligands. 5.Provide new mechanistic insights into copper-mediated cofactor biogenesis in the amine oxidases and galactose oxidase, both of which have unusual organic cofactors that are post-translationally modified in reactions involving Cu and O2 . In particular, studies of the activation of coordinated phenolate and/or thiophenolate ligands will be performed.